Unlike tinyAVR 1-series with more than 8 pins, tinyAVR 0-series cannot drive an external crystal oscillator. As a result, the Real Time Counter (RTC) and the system clock do not have a crystal oscillator clock option nor the option to use an external clock connected to the TOSC1 pin. It is still possible to use an external clock, connected to the EXTCLK pin, as a source for the RTC and system clock.